The invention relates to a direct positive silver halide photographic material, and further detailedly relates to a photographic material which has an internal latent imagetype silver halide emulsion layer that provides a direct positive image by a fogging treatment (for example, an overall exposure, or surface developing treatment in the presence of a fogging agent) after an imagewise exposure.
Conventionally known methods to directly obtain a positive image are roughly classified into two types. One type is the system in which the positive image is obtained after development by using a silver halide emulsion which preliminarily provided fog specks, and destroying fog specks or latent image of the exposed zone with the use of solarization or the Hershell effect. The other type in the system in which the positive image is obtained by fogging treating (treatment for developing speck formation) after the imagewise exposure and then surface developing, or by surface developing during the fogging treatment (treatment for developing speck formation) after the imagewise exposure.
The latter method of the fogging treatment (treatment for developing germ formation) is achieved by an overall exposure, a chemical treatment with a fogging agent, a treatment with a strong developer, or a heat treatment.
The method of the latter type among two methods for forming positive images mentioned above is in general capable of providing photographic materials having a higher sensitivity, compared with the method of the former type and is suitable for uses where high sensitivity is required.
In this technical field, various means have been known, including methods described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,761,266, 3,761,276, and 3,796,577; and British Patent No. 1,151,363.
While photographic materials for formation of positive images can be manufactured by using these well-known technical means at present, further improvement of photographic properties is desired to apply these photographic materials to various photographic fields.
For example, as disclosed in U.S. Pat. Nos. 3,761,267 and 3,206,313, a higher sensitivity is acquired by chemically sensitizing the inside of silver halide particles, or by using a core/shell-type emulsion doped with polyvalent metallic ions. However, such an emulsion has a drawback of low image density. In U.S. Pat. No. 3,761,276, it has been disclosed that a certain extent of chemically ripening treatment is made on the surface of silver halide particles in order to improve the above drawback of low image density. However, such an image is disadvantageous because of a relatively high minimum density, an extremely low stability of the silver halide emulsion during a long term presevation, and a low manufacturing stability.
On the other hand, with a silver halide emulsion mainly composed of silver chloride, which has been disclosed in Japanese Patent O.P.I. Publication No. 32820/1972, the obtained positive image has a relatively high maximum density, however its minimum density is not sufficiently low, and results in unsharpness.
Therefore, in order to fully utilize direct positive photographic materials suitable for the above latter method of the fogging treatment, it is desired to realize an interval latent image-type silver halide emulsion which gives an image with a sufficiently high maximum density and a sufficiently low minimum density, and is excellent in its preservative stability.